Put down that fork: studies document hazards of obesity.

Being overweight or obese in middle age hikes a person's risk of severe heart and kidney problems later in life, even in people whose blood pressure is normal, two new studies show. The reports provide empirical, if observational, evidence that carrying excess pounds causes long-term damage to the body's organ systems.

In one study, researchers analyzed lifestyle and health data--including weight--collected between 1967 and 1973 from adults in the Chicago area. Periodically until 2002, the scientists got medical updates or death information for about 18,000 of the participants who hadn't had heart disease or diabetes when they enrolled about 3 decades earlier. On the basis of their initial checkups, those people were classified as normal weight, overweight, or obese.

Among people with no other heart-risk factors, obese people were four times as likely to be hospitalized for heart problems after age 65 as people of normal weight were, the researchers report in the Jan. 11 Journal of the American Medical Association. Among participants with moderate health risks--including blood pressure at the high end of normal and total cholesterol just over 200--the risk of being hospitalized for heart problems was double in the obese group compared with the normal-weight people, says study coauthor Lijing L. Yan, an epidemiologist at Northwestern University School of Medicine in Chicago and at Beijing University.

Death records of study participants who died after age 65 showed that people who were obese at their initial checkups were also more likely to die from heart attacks or strokes than thinner people were.

"This is the definitive study linking obesity to cardiovascular disease," says S. Jay Olshansky, a gerontologist at the University of Illinois at Chicago.

In a second study, using a health database of northern California residents, scientists at the University of California, San Francisco (UCSF) teamed with researchers at Kaiser Permanente in Oakland to look at the effect of body weight on kidney health. They found that people in the overweight category at checkups conducted between 1964 and 1985 were twice as likely as normal-weight people to have developed kidney failure 15 to 35 years later.

Compared with the normal group, the obese people showed threefold-to-sevenfold the risk of kidney failure, with the most-obese individuals having the greatest risk, says UCSF nephrologist Chi-yuan Hsu. The researchers compared people of similar age, race, gender, smoking habits, cholesterol readings, and heart health. The kidney-disease risk remained strong after the team accounted for diabetes status and blood pressure.

The report appears in the Jan. 3 Annals of Internal Medicine.

Each kidney has about 1 million blood-filtering units called nephrons. As body fat increases, each nephron must handle a greater volume of blood, Hsu says. "This overworking of the nephrons wears them down," he says, perhaps contributing to kidney failure.

Olshansky notes that people in these studies typically became obese in adulthood. "Today, we have many people acquiring obesity as children," he says. If gaining weight in middle age leads to health problems, then getting fat in childhood "might be far worse," he hypothesizes.